Fig. 1. (A) Schematic representation of G-quadruplex (via aptamer folding) directed hemin recognition and fabrication of Au/Ag DONAs – (i) DNA origami nanofork with aptamer in the bridge (zoom-in view of aptamer in box); (ii) folded G-quadruplex structure in presence of K⁺ ions in the bridge where nucleobase(s) in blue, grey, red and yellow represent cytosine, thymine, adenine and guanine, respectively. The backbone is represented as the yellow line connecting the bases; (iii) complexation of hemin (represented in green) with the folded G-quadruplex in the bridge – the dashed brown line represents the electrostatic interaction between Fe³⁺ centre with negative charge of oxygen in the phosphate of sugar-phosphate backbone. (iv) Nanoparticle (Au/Ag) coated with differently functionalised DNA sequence hybridized with DNA capture strand (end labelled with red spheres for representation) hanging out from the upper arms and bridge position (blue spheres) of the DNA origami nanofork to form DONAs. Brown box shows the zoom in top-view of the G-quadruplex–hemin complex bound at the plasmonic hot-spot of DONAs. (B) AFM image of the folded nanoforks with PS2.M aptamer in the bridge, with DNA bridge clearly visible (inset shows the close up view of a DNA origami nanofork with DNA bridge). (C) Representative AFM image of a Au DONA.